1. Field of the Invention
The present invention relates to the automatic lamination of circuit boards or other substrates with a dry film of photopolymer resist material, and more particularly, to a tacking and laminating method and apparatus that enables an increased speed of lamination with greater reliability of film adherence to the substrates.
2. Description of the Prior Art
Dry films of curable photopolymer resist material have been developed for use in the manufacture of printed circuit boards. Such boards have particular use in the electronics industry. A particularly advantageous form of such film that is available commercially is disclosed in U.S. Pat. No. 3,887,450, entitled "PHOTOPOLYMERIZABLE COMPOSITIONS CONTAINING POLYMERIC BINDING AGENTS." U.S. Pat. No. 3,887,450 was granted to Michael N. Gilano et al. and is assigned to the assignee of the present invention. As manufactured, the dry film photopolymer is characterized in being easily contaminated. It is also tacky and limp. Although tacky, the film is "peelable." This makes it possible to use polyethylene cover film for protection and for facilitating handling. The composite is packaged in reels. Typically, such dry films of photopolymer have thicknesses of 1.0, 1.5, 2.0, and 3.0 mils, and have widths in the range of six (6) inches to thirty (30) inches.
Machines, termed laminators, are known in the prior art for applying dry films of curable photopolymer to the surfaces of each of a plurality of printed circuit boards in succession. In one form thereof, such laminators utilize a so-called two-step laminating process consisting, in a first step, of pressing in the presence of heat, the leading end of a film sheet into pressure sealed contact with the face of the board at a forward portion thereof, a step referred to hereinafter as "tacking;" and in a second step, feeding the board, with the film sheet trailing behind, between heated pressure or lamination roller means for compression bonding of the film sheet onto the face of the board, a step referred to hereinafter as "laminating."
Thus, as disclosed in U.S. Pat. Nos. 3,658,629 to L. P. Cramer et al., 4,025,380 to E. T. Bernardo 4,650,536 to A. B. Ceraso and 4,659,419 to E. Miyake, each of a plurality of blank boards is transferred, in turn, to a film web tacking station of the laminator where the leading end of a continuous length of film web is tacked to a forward end portion of the board. The board is then transferred to and moved through a laminating station, with the film web being pulled along, where lamination rolls cause the film web to become bonded to the board. During the laminating operation, the film web is cut to form a film portion of predetermined length that matches the length of the board.
Such film sheet portions may simultaneously be applied to the opposite sides of the board with each film portion being of the same, larger or smaller size than that of the board being laminated. As disclosed in the aforementioned U.S. Pat. No. 4,650,536 to A. B. Ceraso, applying a smaller film sheet portion which fits within the perimeter of the board advantageously leaves free the edges thereof in which centering holes may be disposed for use in subsequent printed circuit board processing operations.
It is further known in the prior art, as disclosed in the aforementioned U.S. Pat. No. 4,659,419 to E. Miyake, to effect the cutting operation of the film web without interrupting the laminating operation. As there disclosed, the film web cutting system consists of a circular rotating cutting blade which is moved across the width of the film web while at the same time being moved toward the laminating station in the same direction and at the same speed as the film web is being moved thereto.
The prior art laminators are characterized in that the portion of the film web at the leading end thereof that is tacked to the board has a length of a few millimeters only. Moreover, the location where the end of the film web is tacked to the board is spaced a distance of several centimeters from where the laminating action begins. Upon completion of the tacking operation, the board is pushed forward under a lamination roll in the laminating station. This action subjects the film web to a pulling force which places the film web under tension. Such tension on the film web creates a problem, giving rise to a critical situation, as there is a tendency for the film web to be pulled away from and off the board from the region where it had been tacked, laminating action not yet having begun, as illustrated herein in FIG. 1 of the drawings.
A critical situation develops also, when a single side only of a board is being laminated. In this case, the pulling force and resulting tension on the film web tends to cause the board to bend. This problem is most pronounced with thin boards, as illustrated herein in FIG. 2 of the drawings.
The tendency for such critical situations to develop with the prior art laminators requires that the tacking be effected under high pressure and temperature for a sufficient time to effect a tight bond and also places a restriction on the magnitude of the force that may be applied to the film web while the board is being advanced toward the laminating station, and hence, on the speed at which the board may be so advanced. These factors detract seriously from the efficiency of lamination and are significant in determining the number of boards that may be satisfactorily laminated in any given period.
Another factor that is a determinant of the efficiency of lamination is the film web cutting time, that is, the elapsed time required for the rotary cutting blade to be moved across the width of the film web to effect the film sheet portion cutting action. With the rotary blade cutting system of the prior art, the elapsed time required for cutting the film web is sufficiently long, being of the order of several seconds, that it also is a significant factor in determining laminating efficiency, not only in respect of reducing the number of boards that may be laminated in a given period but also in respect of restricting the shortest length of board that may be laminated in the machine to a size larger than desired.
Thus, there exists a need and a demand for improvement in the method of and apparatus employed for tacking, laminating, and film cutting to the end of achieving greater efficiency and reliability of lamination. The present invention was devised to fill the technological gap that has existed in the art in these respects.